FIG. 1 shows a typical optical lithographic fabrication system 100 for delineating features in a workpiece 120. Typically the workpiece 120 comprises a semiconductor wafer (substrate), together with one or more layers of material(s) (not shown) located on a top major surface of the wafer. More specifically, typically monochromatic optical radiation of wavelength .lambda. emitted by an optical source 106, such as a mercury lamp, propagates successively through a pinhole aperture 15 in an opaque screen 105, an optical collimating lens or lens system 104, a patterned lithographic mask or reticle 103 having a pattern of apertures (object features in the form of bright regions) in an opaque material, and an optical focusing lens or lens system 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the workpiece 120. Thus the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a development process, typically a wet developer, the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on") the photoresist layer 101. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the workpiece 120. Portions of the workpiece 120 thus are removed from the top surface of the workpiece 120 at areas underlying those where the photoresist layer 101 was removed by the development process but not at areas underlying those regions where the photoresist remains. Alternatively, instead of etching the workpiece, impurity ions can be implanted into the workpiece 120 at areas underlying those where the photoresist layer was removed by the development process but not at areas underlying where the photoresist remains. Thus, in an event, the pattern of the mask 103--i.e., each feature of the mask--is transferred to the workpiece 120 as is desired, for example, in the art of semiconductor integrated circuit fabrication.
As known in the art, the aperture 15 is located on the focal plane of the collimating lens 104, and the indicated distances L1 and L2 satisfy in cases of a simple lens 102: 1/L1+1/L2=1/F, where F is the focal length of the lens 102.
In fabricating integrated circuits, it is desirable, for example, to have as many transistors per wafer as possible. Hence, it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width--or of an aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization and another.
According to geometric optics if it is desired to print on the photoresist layer 101 the corresponding image feature having a width equal to W, an object feature having a width equal to C must be located on the mask (reticle) 103. Further, according to geometric optics if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L 1, and where m is known as the lateral magnification. When diffraction effects become important, however, instead of a sharp black-white image a diffraction pattern of the object feature C is formed on the photoresist layer 101, whereby the edges of the image become indistinct; hence the resolution of the features of the reticle 103, as focused on the photoresist layer and transferred to the workpiece, deteriorates.
Another problem, caused by diffraction, is "proximity effects"; that is, the actual printed width of an object feature of width L on the mask depends upon the distance (spaces or gaps, G) between neighboring object features on the mask. This problem arises in cases where L.ltoreq.(0.7).lambda./(NA), approximately, and at the same time G/L.gtoreq.2, approximately, and where NA is the numerical aperture on the image side of the system 100.
Another consideration that arises in the system 100 is that of depth of focus (hereinafter: "DOF") of the image features on the photoresist layer 101 of the feature being imaged. If, for example, the DOF is less than the thickness of the photoresist (hereinafter: "PR") layer 101, then the edges of the image of the feature will be indistinct, and hence subsequent development of the PR layer will result in a feature therein whose sidewalls are undesirably not vertical. As a result, the dimensions of a printed feature deviates from the intended desired value. Moreover, in cases where the top surface of the wafer is not planar by an amount not insignificant as compared with the DOF, even if the thickness of the PR layer is less than the DOF, non-vertical sidewalls of the developed feature in the PR layer after development will likewise result, and an indeterminacy in the position of the sidewalls will also occur.
In prior art, in order to increase the DOF of a phase-shifting mask, the optical intensity transmission T of the opaque portions of the mask was set at a value in the approximate range of 0.05 to 0.15 typically at approximately 0.10, rather than zero. However, then the side lobes of the diffraction pattern are undesirably increased in intensity, at the expense of the central lobe. Also, OFF-axis (FIG. 2) illumination (instead of the previously described ON-axis illumination) was used to enhance the imaging by reason of not only increasing the DOF but also improving the magnitude of the maximum relative slope (dI/dx)/I in the central diffraction lobe of the image--where I=optical intensity, and x=horizontal distance along the PR layer.
An example of OFF-axis illumination is shown in the system 200 (FIG. 2) where, instead of a single pinhole aperture 15 in the screen 105, a thin annular aperture 25 subtending an angle equal to 2.alpha. at the lens 104 is used. Typically, .alpha.=45.degree.(=.pi./4 radian), approximately. Alternatively, a pattern of apertures can be used to provide OFF-axis illumination, as known in the art. Parenthetically: the larger the magnitude of the maximum value of (dI/dx)/I in the central diffraction lobe, the more reproducible will be the printing of the image formed on the PR layer.
The use of OFF-axis illumination, however, thus improved the DOF only for some values of, for example, the width L of a line feature printed on the PR layer, where a "line feature" refers to a feature in which its length is equal to at least three times its width L. More specifically, although OFF-axis illumination enhanced the DOF for cases in which L was equal to or less than approximately (0.75).lambda./(NA) and at the same time the spaces G between adjacent line features was also equal to or less than approximately (0.75).lambda./(NA), nevertheless OFF-axis illumination degraded both the DOF and the (dI/dx)/I of the image for cases in which L was equal to or greater than (0.75).lambda./(NA). Thus in practical situations, especially random-layout logic chips--in which there are some image features for which L is less than or equal to approximately (0.75).lambda./(NA) and there are other image features for which L is greater than or equal to approximately (1.0).lambda./(NA)--OFF-axis illumination gives rise to its own DOF problem. Also, it is desirable to counteract the sidelobe problem caused by setting T unequal to zero in a phase-shifting mask, which in turn was desirable to improve diffraction limitations. It is further desirable to set T unequal to zero in order to make the mask more useful for the more random layouts.